With the increasing complexity of distributed data processing systems, particularly those utilizing telephone data communication between central and remote Data Terminal Equipments (DTE) and their associated Data Circuit Terminating Equipments (DCE), the need for testing and control of these DCE's has increased. DCE malfunctions become increasingly critical in that one malfunctioning DCE may interrupt transmission by many others in the network. The need has arisen to automatically control DCE's in a distributed system to minimize time losses Therefore, it has become highly desirable to provide the DCE with the capability to send alarm signals in case of failure, to make the difference between a failing link, and a failing remote DCE.
In analog data communication networks the Data Circuit Terminating Equipment is a so called modem which modulates the digital data transmitted by the Data Terminal Equipment, and demodulates the data received from the communication network for the associated DTE. Each remote modem is generally provided with an alarm device, able to send an alarm tone to the control station when the modem is out of order because of a power supply failure or of an internal hardware failure. Such an alarm tone is, in an analog network, a special signal different from the line modulation.
The trend is now to install digital communication networks instead of analog communication networks. A network of this type is the Dataphone Digital Service (DDS), already in use in the United States. In such a digital network, the Data Circuit Terminating Equipment (DCE) is called a Data Service Unit (DSU).
In a digital communication network, the alarm is given by sending a bit configuration which has two characteristics it must be recognized as alarm signals and it must have the priority over any other data bit configuration being transmitted at the same time. On the DDS network, the bit configuration which fulfills the latter conditions is a long sequence of 0 bits. This is based upon the fact that the network behaves as an AND circuit when it is receiving, on a common line, several messages from different remote stations in a multipoint configuration. It must be noted that such a bit configuration would be a sequence of 1 bits if the network would behave as an OR circuit rather than an AND circuit.
Assuming that a certain bit configuration (such as a sequence of 0bits or of 1 bits) be transmitted as a digital alarm signal by the Data Service Unit in distress, the problem is to avoid the initiation of this signal during normal DTE to DTE communications over the network.